Method for fabricating an epitaxial substrate

ABSTRACT

A method for fabricating an epitaxial substrate. The technique includes providing a crystalline or mono-crystalline base substrate, implanting atomic species into a front face of the base substrate to a controlled mean implantation depth to form a zone of weakness within the base substrate that defines a sub-layer, and growing a stiffening layer on a front face of the base substrate by using a thermal treatment in a first temperature range. The stiffening layer has a thickness sufficient to form an epitaxial substrate. In addition, the method includes detaching the stiffening layer and the sub-layer from the base substrate by using a thermal treatment in a second temperature range higher than the first temperature range. An epitaxial substrate and a remainder of the base substrate are obtained. The epitaxial substrate is suitable for use in growing high quality homoepitaxial or heteroepitaxial films thereon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending Internationalapplication no. PCT/______ filed May 19, 2004, and acontinuation-in-part of copending U.S. application Ser. No. 10/716,901,filed on Nov. 18, 2003, the entire contents of which are expresslyincorporated herein by reference thereto.

BACKGROUND ART

The invention relates to a method of fabricating an epitaxial substratewhich is suitable for use in fabricating good quality homo- orhetero-epitaxial films or layers.

Epitaxy is a process during which a crystalline layer of a material isdeposited on a substrate, which is also crystalline. Epitaxial growth ischaracterized by having the crystalline structure of the substratereproduced in the epitaxial layer of the material that is grown.Consequently, the defects present in the substrate are usuallyreproduced in the epitaxial layer. Epitaxial layers are typically usedin electronic or optoelectronic applications. Of particular interestare, for example, gallium nitride epitaxial layers which, due to theirlarge band gap, are used in blue, violet or ultraviolet laser diodes.

Epitaxial techniques can be essentially grouped into two families.First, there is homoepitaxy, wherein the material to be grown is of thesame nature as the substrate. This means that the crystallographicstructure and the chemical nature of the substrate and the resultinglayer are essentially identical. Typical examples used in industry arethe homoepitaxy of silicon on a silicon substrate, or the epitaxialgrowth of gallium arsenide on a substrate of gallium arsenide.

Of even greater interest is heteroepitaxy, wherein a film layer is grownon a substrate of a different nature. This is especially important wherethe desired material is not available in the form of a crystallinesubstrate. There are two major problems inherent with heteroepitaxy: thedifference in the crystalline structure of the two materials, and thedifference in their thermal expansion coefficients. These differenceslead to stress inside the film and consequently to defects such asdislocations.

In addition to the above-mentioned problems that result in insufficientepitaxial layer quality, it is also possible that the substrate, due toits intrinsic characteristics, is not suitable for a particular deviceapplication that would use the special characteristics of the epitaxiallayer. For example, when growing gallium nitride on silicon carbiderelatively good epitaxial growth could be achieved. However, if thegallium nitride structure is intended for fabrication of alight-emitting device, the silicon carbide substrate would not beadvantageous, as it would trap too much light.

Several approaches are known for overcoming the above-mentionedproblems. However, none proposes a substrate which allows homo- as wellas heteroepitaxial Growth and overcomes each of the three distinctproblems mentioned above. For example U.S. Pat. No. 5,759,898 disclosesthat it is possible to successfully grow an epitaxial silicon germaniumthin film on a silicon-on-insulator wafer. In this case, the silicongermanium layer grows on a thin silicon film (about 16 nm), which itselfis positioned on a layer of silicon dioxide, which is in turn positionedon a silicon wafer. In such a structure, the appearance of dislocationsin the silicon germanium film is greatly diminished in comparison to asilicon germanium film directly grown on a bulk silicon substrate.

Another approach is described in International Publication WO 99/39377,wherein a “compliant” substrate is created by ion implantation. In thisapproach, ion implantation is used to obtain a weakened layer inside thematerial, which also creates a very thin layer on the top. This toplayer is partially isolated from the substrate via the weakened layer,and to some extent absorbs the crystalline structure and thermalexpansion mismatch between the top layer of the original substrate andthe epitaxial layer to be grown, thereby causing at least a partialrelaxation of the stress of the epitaxial layer.

These two approaches overcome to some extent the above-mentionedproblems. However, the presence of the original substrate imposes anegative influence that is not excluded. In some cases the presence ofthe original substrate is incompatible with the desired application ofthe epitaxial layer that is grown, so that further processing steps areneeded to remove the original substrate, which leads to higherproduction costs. Thus, improvements in these products and the processesof making them are desired.

SUMMARY OF THE INVENTION

A method for fabricating an epitaxial substrate is presented. Thetechnique includes providing a crystalline or mono-crystalline basesubstrate, implanting atomic species into a front face of the basesubstrate to a controlled mean implantation depth to form a zone ofweakness within the base substrate that defines a sub-layer, and growinga stiffening layer on a front face of the base substrate by using athermal treatment in a first temperature range. The stiffening layer hasa thickness sufficient to form an epitaxial substrate. In addition, themethod includes detaching the stiffening layer and a sub-layer from thebase substrate by using a thermal treatment in a second temperaturerange higher than the first temperature range. An epitaxial substrateand a remainder of the base substrate are obtained, and the epitaxialsubstrate is suitable for use in growing high quality homo-epitaxial orhetero-epitaxial films thereon. The epitaxial substrate is thereforesufficiently robust to withstand detachment from the remainder of thebase substrate, and/or sufficiently stable that it can be detached fromthe remainder of the substrate by appropriate means, for example, inorder to place it in a different apparatus for further processing.

Advantageously, the atomic species used during the implanting step maybe at least one of hydrogen ions and rare gas. Implanting atomic speciesis a very elegant way of defining the area where, in a later processingstep, detachment takes place between the epitaxial substrate and theremainder of the base substrate.

In a preferred embodiment, the implanting step occurs before growing thestiffening layer. It is known that during ion implantation the implantedions do not only disturb the crystalline structure on the desired levelinside the substrate, but that defects are also introduced into thelayer through which the ions travel. This means that if ion implantationwere to take place after providing the stiffening layer, the stiffeninglayer would contain defects that might influence the quality of theepitaxial layer to be grown. Therefore, it is advantageous to performion implantation first and thereafter grow the stiffening layer.

In an advantageous implementation of the invention, the sub-layer isless than approximately 5 μm thick, preferably less than 2 μm thick andmore preferably less than 1 μm thick. Choosing a relatively thicksub-layer has the advantage that the thickness of the stiffening layercan be relatively small, thereby speeding up the process. However,choosing a relatively thin sub-layer results in minimizing anyundesirable properties of the sub-layer material, for example, poorlight absorbent properties for optical applications. Thickness controlof the sub-layer can easily be managed by adjusting the energy of theions.

The first temperature range may beneficially be in the range from aboutroom temperature to about 900° C., and the second temperature range maybeneficially be in the range of from about 900° C. to about 1100° C. Dueto the thermal treatment in the second temperature range, the weakenedzone inside the base substrate is further weakened, which leads to arupture between the remainder of the base substrate and the stiffeninglayer together with the sub-layer. Thermal treatments are generally easyto implement and to control. If necessary, a mechanical treatment can beapplied to attain the final detachment.

In another advantageous implementation according to the invention, asacrificial layer is provided on the front face of the base substrateprior to implanting atomic species. The sacrificial layer may be a thinsilicon dioxide (SiO₂) layer. In an embodiment, the sacrificial layer isremoved before providing the stiffening layer. The sacrificial layerhelps to protect the substrate surface from typical organic and particlecontamination, which can occur during high-energy ion implantation. Inaddition, when a sacrificial layer is used, any contamination is fixedon this layer, and subsequent removal of the sacrificial layer alsoremoves the contamination. Furthermore, the crystallographic propertiesor the thermal expansion coefficient of the sacrificial layer might notbe suitable for growing the desired stiffening layer or epitaxial layer.

In a preferred embodiment, the stiffening layer is grown by at least oneof epitaxial growth, molecular beam epitaxy (MBE), metal-organicchemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE) orby sputtering. In addition, the stiffening layer may be in the range ofapproximately 5 μm to at least approximately 50 μm thick. Such athickness of the stiffening layer together with that of the sub-layerprovides sufficient stability to the epitaxial substrate so that itssurface does not become deformed or destroyed when detached from thebase substrate.

In another advantageous implementation of the invention, the surface ofthe base substrate is pre-treated prior to growing the stiffening layerby using at least one of HF etching, plasma etching, or a standardcleaning treatment. Such pre-treatments improve the quality of thestiffening layer.

In a variation, at least one additional layer is provided on top of thestiffening layer or between the base substrate and the stiffening layer.The additional layer between the base substrate and a stiffening layermay be a buffer layer, the buffer layer being made of at least one ofAlN, GaN, AlGaN or a combination thereof. Such layers are helpful inovercoming the residual differences in crystallographic structure andthermal expansion. Therefore, by applying these kinds of additionallayers, residual stresses can be further diminished, leading to improvedepitaxial films. These materials can also be arranged in the form ofseveral layers.

In yet another beneficial implementation, at least two additional layersare provided on top of the base substrate, wherein at least one of theadditional layers is provided prior to implanting atomic species. Inaddition, the atomic species may be implanted into the at least oneadditional layer to create a weakened zone inside the at least oneadditional layer. This means that after the detachment step, no materialfrom the original substrate is present and thus the residual stress canbe further diminished, resulting in improved epitaxial films. In thiscase, the remainder of the base substrate is made of the entire basesubstrate itself and the part of the at least one additional layersituated between the weakened zone and the base substrate.

In a variation according to the invention, a surface of the remainder ofthe base substrate is polished after detaching the epitaxial substratesuch that the base substrate is suitable for reuse. This permits anon-negligible amount of high quality epitaxial substrates to bemanufactured from a relatively expensive mono-crystalline basesubstrate. The base substrate may be made of at least one of silicon,silicon carbide, sapphire, gallium arsenide. indium phosphide (InP) orgermanium (Ge). For epitaxial growth, these substrates are usuallymono-crystalline and can present surfaces of various crystallinedirections. These substrates are readily available and because theirproperties vary, they form a good base for growing a large number ofdifferent epitaxial substrates, thereby permitting the growth of an evenlarger amount of epitaxial layers with good crystalline properties.

In a preferred embodiment, the stiffening layer is epitaxially grown andis made of the same material as an epitaxial film to be grown on theepitaxial substrate in a subsequent fabricating step. The stiffeninglayer may have a crystalline structure and a thermal expansioncoefficient similar to that of an epitaxial film to be grown on theepitaxial substrate in the subsequent fabricating step, and could bemade out of at least one of gallium nitride (GaN), aluminum nitride(AlN), indium nitride (InN), silicon germanium (SiGe), indium phosphite(InP), gallium arsenide (GaAs) or alloys made out of those materials.Suitable alloys include at least one of AlGaN, InGaN, InGaAs or AlGaAs.These materials are particularly suitable for electronic as well asopto-electric applications and can be arranged in the form of severallayers within the stiffening layer. Moreover, the stiffening layermaterial can be advantageously chosen based on the properties of thebase layer and the desired epitaxial film layer to be grown later, sothat any crystallographic structure or thermal expansion coefficientdifferences are minimized. Furthermore, any strain that may occur duringgrowth of the epitaxial film layer can be absorbed to a large extentwithin the epitaxial substrate.

The method advantageously includes growing a homoepitaxial layer or aheteroepitaxial layer on the epitaxial substrate, which may be grown byusing a thermal treatment in a third temperature range that is higherthan the first temperature range. The third temperature range thusincludes temperatures greater than about 900° C. The homoepitaxial layeror the heteroepitaxial layer that is grown may be made out of at leastone of GaN, SiGe, ALN or InN. Such a homoepitaxial layer or theheteroepitaxial layer is of good quality and may be used, for example,to fabricate electronic devices such as light emitting diodes.

An epitaxial substrate produced by following the methods according tothe invention is economical to produce, and is minimally influenced bythe material of the base substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, purposes and advantages of the invention will becomeclear after reading the following detailed description with reference tothe attached drawings, in which:

FIGS. 1 a to 1 e schematically show the method of preparing a epitaxialsubstrate according to the present invention, which may be used tofabricate a homo- or heteroepitaxial film; and

FIGS. 1 f to 1 h schematically show additional processing steps of asecond embodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Processes for preparing an epitaxial substrate are described herein. Inan embodiment, a crystalline base substrate is provided, a stiffeninglayer is grown on the face of the base substrate using thermal treatmentin a first temperature range, and the stiffening layer is detached alongwith a sub-layer of the base layer by using a second thermal treatmentin a second temperature range that is higher than the first temperaturerange. The resulting epitaxial substrate is suitable for growing highquality homoepitaxial or heteroepitaxial films thereon. The stiffeninglayer may be detached from the base layer along a zone of weaknesscreated by implanting ions into the base substrate.

In a preferred embodiment, the stiffening layer is epitaxially grown andis made of the same material as the homo or heteroepitaxial layer to begrown in a subsequent fabricating step, such that the homo orheteroepitaxial layer grows in a homoepitaxial growth mode. This meansthat between the epitaxial film and the epitaxial substrate there isnearly no difference in crystalline structure and thermal expansioncoefficients, resulting in high quality epitaxial films. In anothervariant, the stiffening layer can have a crystalline structure and/orthermal expansion coefficient similar to that of the homo orheteroepitaxial layer so that it grows in a heteroepitaxial growth mode.When the difference in crystalline structure or thermal expansioncoefficient of the base substrate and the desired epitaxial film wouldbe too great, it is advantageous to choose a stiffening layer of amaterial that has its properties somewhere in between those two values.By doing so, a good crystalline quality of the stiffening layer can beobtained and also a good quality epitaxial film can be realized insubsequent fabrication steps.

The stiffening layer could be made out of at least one of galliumnitride (GaN), aluminum nitride (AlN), indium nitride (InN), silicongermanium (SiGe), indium phosphite (InP), gallium arsenide (GaAs) oralloys made out of those materials. In addition, the alloys couldinclude at least one of AlGaN, InGaN, InGaAs or AlGaAs. These materialsare particularly of interest for electronic as well as opto-electronicapplications, and can be arranged in the form of several layers withinthe stiffening layer.

In order to further improve crystalline quality of the epitaxialsubstrate, the temperature ranges during which the stiffening layer isgrown, during which detachment takes place, and during the growth of thehomo- or heteroepitaxial film, are chosen in the following manner. Thefirst temperature range is chosen such that epitaxial growth is achievedand no change of a weakened zone occurs which could deform or destroythe epitaxial substrate. Once the stiffening layer has grownsufficiently thick, growth is stopped and the temperature is raised tofurther weaken the weakened zone so that detachment occurs to create theepitaxial substrate. In a variant, instead of using thermal energy,mechanical energy may be applied to achieve detachment at the weakenedinterface. Lastly, after the detachment step, the temperature is chosenso that the growth conditions for the epitaxial layer are optimized toachieve improved crystalline quality. Thus, prior to detachment thetemperature is chosen to balance crystalline growth and to preventdetachment, and after detachment the temperature can be adapted to thegrowth parameters only.

Furthermore, according to an advantageous embodiment, the epitaxiallayer can be grown in a third temperature range that is a highertemperature than the first temperature range. Thus an optimized growthtemperature can be chosen which may be higher than the growthtemperature of the stiffening layer, without having to be concernedabout stress developing in the remainder of the base substrate.

In a further preferred embodiment, implantation of ions takes placebefore providing the stiffening layer. It is known that during ionimplantation the implanted ions do not only disturb the crystallinestructure on the desired level inside the substrate, but also introducedefects into the layer through which the ions travel. Thus, if ionimplantation were to take place after providing the stiffening layer,the stiffening layer would contain defects that might detrimentallyaffect the quality of the epitaxial layer to be grown. Therefore, it isadvantageous to perform ion implantation first and then to grow thestiffening layer.

Advantageously the first temperature range can be from about roomtemperature to 900° C., and more specifically to 800° C. The secondtemperature range can be from about more than 900° C. to about 1100° C.,and/or the third temperature range may start at temperatures of morethan about 900° C. Use of these temperatures results in epitaxialsubstrates having an improved crystalline quality, in particular forGaN.

In a further preferred embodiment, the base substrate is made out of oneof the group of silicon, silicon carbide, sapphire, gallium arsenide,indium phosphide (InP) or germanium (Ge). For epitaxial growth, thesesubstrates are usually mono-crystalline and can present surfaces ofvarious crystalline directions. These substrates are readily availableand because their properties vary, they form a good base for growing alarge number of different epitaxial substrates, thereby permitting thegrowth of an even larger amount of epitaxial layers or films with goodcrystalline properties.

In a preferred embodiment, an heteroepitaxial film is grown on theepitaxial substrate. The heteroepitaxial film may be made out of alleast one of GaN, SiGe, ALN or InN.

In a variant, additional or several layers are provided between the basesubstrate and the stiffening layer of gallium nitride or the like. It isespecially advantageous for such a buffer layer to be made of one ofseveral of AlN, GaN, AlGaN or a combination thereof, depending on thematerials of the base substrate and the stiffening layer. By using thesematerials as buffer layers, constraints that eventually occur can belimited, resulting in an improved quality of the epitaxial films to begrown on the epitaxial substrate. These materials can be arranged in theform of several layers.

In another advantageous implementation of the invention, a backsidesurface of the epitaxial substrate created after the detaching step hasa surface roughness in a range of approximately 20 to about 200 Å RMS.The range may be between approximately 20-150 Å RMS, and morespecifically in a range of approximately 20-100 Å RMS. Thus, even thoughthe epitaxial substrate is separated from the remainder of thesubstrate, there will be sufficient friction between the epitaxialsubstrate and the remainder of the base substrate so that the positionof the epitaxial substrate can be relatively well maintained. Therefore,it is possible to continue to process the structure in the sameapparatus by growing the next layer. This could be another buffer layeror the final desired homo or heteroepitaxial layer, and it could begrown without having to handle the epitaxial substrate in a specificway, for example by securing it to a substrate holder. Therefore, it ispossible to continue growing the next layer in the same depositionapparatus which could be another buffer layer or the final homo orheteroepitaxial layer.

Consequently, the present method which includes detaching the epitaxialsubstrate from the remainder of the base substrate solves the problemsof the prior art. In cases where a commercially available base substrateis not compatible with the desired application of an epitaxial film tobe grown, the present invention minimizes the incompatibility becausethe epitaxial substrate is independent from the remainder of the basesubstrate, and therefore is not affected by the undesirable properties.The influence of any sub-layer that may be present is also minimized.

With regard to the problems of crystallographic structureincompatibility between the base substrate and the desired epitaxiallayer, and also the differences in their thermal expansion coefficients,the creation of a epitaxial substrate according to the invention is alsoadvantageous, because the stiffening layer provided on top of the basesubstrate is itself grown by a heteroepitaxial method. This means thatby choosing the right material for the stiffening layer, which choicedepends on the properties of the base material and the desired epitaxialfilm, any incompatibilities can be reduced. Furthermore, strain thatmight occur during the growth of the desired epitaxial layer on top ofthe epitaxial substrate can be absorbed to a large extent inside theepitaxial substrate. When the difference in crystalline structure orthermal expansion coefficient of the base substrate and the intendedepitaxial film would be too great, it is advantageous to choose astiffening layer material that has properties somewhere in between thesetwo values. Thus, by following the present inventive process, epitaxialsubstrates are obtained that provide for growth of better qualityepitaxial films. The invention also relates to an electronic device,manufactured on or within the epitaxial substrate fabricated accordingto the described process.

FIGS. 1 a-1 h illustrate the process which includes using, for example,a SMART-CUT® transfer process production line.

FIG. 1 a shows a base substrate which acts as the starting point forpreparing an epitaxial substrate according to an embodiment of thepresent process. This substrate 1 has a front side 2 and a back side 3.For homo- or heteroepitaxial applications, the base substrate is made ofa crystalline material, in particular a mono-crystalline type material.At least one surface, here the front side 2, is sufficientlywell-defined to serve as a starting point for the growth of anepitaxial, homo- or heteroepitaxial layer or film. The surface istypically polished and prepared for epitaxy, substantially in accordancewith typical crystalline directions. The exact size of the basesubstrate 1 is not of major importance, but at least the surface of thefront side 2 should be sufficiently large enough to allow the growth ofa sufficiently large epitaxial substrate suitable for the subsequentneeds of devices to be fabricated. Particularly relevant are wafers madeout of silicon, silicon carbide, sapphire, gallium arsenide, indiumphosphide or germanium. These wafers are usually disc-shaped, butrectangular or square formats are also available. Disc-like wafers canhave diameters of from 50.8 mm (2 inches) up to 300 mm, and even largerdiameter wafers are foreseeable in the future.

FIG. 1 b shows the result of the next processing step. In thisembodiment, gaseous species, in particular hydrogen ions, are implantedthrough the front side 2 of the base substrate 1. The implanted speciescreate a zone of weakness 4, which is essentially parallel to thefront-side surface 2 of the base substrate 1. The depth 5 of the zone ofweakness 4 can be adjusted by controlling the energy of the implantedspecies. For example, in the case of hydrogen atoms implanted intosilicon carbide, an implantation energy of approximately 100 KeV createsa zone of weakness at a depth of approximately 6000 Å. The portion ofmaterial between the surface 2 and the zone of weakness 4 defines thesub-layer 6. It is particularly advantageous to utilize a SMART-CUT®process to achieve the ion implantation.

FIG. 1 c shows the step of depositing a stiffening layer 7 on the basesubstrate. It should be noted that, at the beginning of the process, thebase substrate 1 has been chosen so that its material composition willbe compatible with epitaxially growing the stiffening layer 7. Inparticular, it should be compatible with heteroepitaxial growth.

In this first epitaxial growth step, the stiffening layer is grown to athickness of approximately 1 to 50 micrometers. The thickness 8 of thestiffening layer 7 plus the thickness 5 of the sub-layer 6 must besufficient to create a epitaxial substrate 10 that is free-standing,which means that the quality of the surface 9 of the stiffening layer 7does not deteriorate at or after the point when the epitaxial substrate10 is detached from the remainder of the base substrate 11. Typicalstiffening layer materials are gallium nitride (GaN), aluminum nitride(AlN), indium nitride (InN), silicon germanium (SiGe), indium phosphite(InP), gallium arsenide (GaAs) or alloys made out of those materials,like AlGaN, InGaN, InGaAs, AlGaAs. However, in future applications itmay be possible for other types of materials to play the role of astiffening layer. Usually, the stiffening layer 7 can be applied by anydeposition method leading to epitaxial growth. However, growth bymolecular beam epitaxy (MBE), metal-organic chemical vapor (MOCVD),hydride vapor phase epitaxy (HVPE) or sputtering is particularlysuitable.

The temperature for growing the stiffening layer 7 is chosen to permitepitaxial growth without permitting detachment at the weakened zone 4,which could adversely affect the quality of the stiffening layer or mayeven lead to the destruction of the stiffening layer. The stiffeninglayer is typically grown at temperatures less than 900° C.

FIG. 1 d shows the result of the next processing step, which is thethermal treatment of the structure that has been created after thegrowth of the stiffening layer 7 has been terminated. The epitaxialsubstrate 10 is detached from the remainder of the base substrate 11.The surface 12 does not deteriorate because the total thickness of theepitaxial substrate 10, being the sum of the thickness 5 and thethickness 8 (see FIG. 1 c), is sufficient to prevent the formation ofdefects due to the morphology of the surface 12 opposite the zone 13where the separation or fracture occurs. The surface 13 roughness wherethe fracture occurs is sufficient to securely hold the epitaxialsubstrate 10 in a position on top of the remainder of the substrate 11due to frictional interaction. This can be advantageous for conductingsubsequent processing steps, in particular when such subsequentprocessing steps are performed in the same apparatus without removingthe epitaxial substrate 10. Typically, roughness values in the range ofapproximately 20-200 Å RMS, more specifically in a range ofapproximately 20-150 Å RMS and even more specifically in a range ofapproximately 20-100 Å RMS, can be achieved by using such a process. Thedetachment process occurs at temperatures higher than that used duringthe growth of the stiffening layer 7. A typical temperature would be900° C.

FIG. 1 e shows the result of a subsequent processing step, which is theepitaxial growth of the desired material leading to a homo orheteroepitaxial film 14 to thus create an epitaxial layer made ofsublayer 6, stiffening layer 7 and homo or heteroepitaxial film 14.Thus, a planned epitaxial film 14 is obtained during this seconddeposition step, which is conducted in a similar manner to the firstone. But during the second epitaxial step, the temperature may beoptimally chosen with respect to epitaxial growth without taking adetachment process into account. Depending on the goal of the process,the material of the second film 14 can be the same as the material ofthe stiffening layer 7 of the base substrate 11. In the alternate,heteroepitaxial growth may occur, wherein the material of layer 14 isdifferent from that of the stiffening layer 7. As the epitaxialsubstrate 10 is detached from the remainder of the base substrate 11,the crystalline quality of this second layer 14 is very good. Defectsdue to the stress can be minimized. Negative effects of the sub-layer 6of the epitaxial substrate 10, which contains the material of the basesubstrate 1, can usually be ignored, as its thickness is usually chosento be clearly thinner than that of the stiffening layer 7. This meansthat the remainder of the base substrate 11 behaves like a bulksubstrate of the material of the stiffening layer 7. This also meansthat the thickness of the second film 14, especially in cases where thematerial of layer 14 and that of layer 7 is the same, is not limited. Itis even possible to grow a film 14 with a laterally variable thickness.The epitaxial substrate 10 does not impose limitations on the structureor thickness of the second layer 14. The process can be applied, forexample, to grow mono-crystals of excellent quality, especially wherethe material of layers 14 and 7 are the same, or heterostructures can bebuilt, leading to devices like laser diodes.

In an optional processing step (not shown), the sub-layer 6 can beremoved, for example by using a dry or wet etch technique. In addition,the remainder 11 of the base substrate 1 can be polished, in particularby chemical-mechanical polishing (CMP), and can be re-used as a basesubstrate 1 in a subsequent epitaxial substrate 10 manufacturingprocess.

FIGS. 1 f to 1 h show a variant of the process described above. Withrespect to the first embodiment of FIGS. 1 a to 1 e, these processingsteps replace the processing step shown in FIG. 1 b. In particular, FIG.1 f shows the result of depositing a sacrificial layer 15 on top of thebase substrate 1. This sacrificial layer 15 is, for example, made out ofsilicon dioxide and may be deposited according to one of the usualmethods known in the art. This sacrificial layer protects the front side2 of the base substrate 1 from organic and particle contamination whichmight occur during the subsequent ion implantation process step.

FIG. 1 g shows the result of the ion implantation step. It is comparableto the process step described in combination with FIG. 1 b. However, dueto the presence of the sacrificial layer 15 the depth 5 of the weakenedzone 4 is smaller than that of the first embodiment. This occurs becauseimplantation is at the same energy level and the gaseous species nowhave to travel through the sacrificial layer as well. As a consequence,the gaseous species stop at a depth 5 in the base substrate which isless than that of the first embodiment.

FIG. 1 h shows the result of removal of the sacrificial layer. After theion implantation, the sacrificial layer 15 is removed in order toachieve a clean front side 2 surface of the base substrate 1. The nextstep is the growth of the stiffening layer which is comparable to whathas been described above with respect to FIG. 1 c.

Several further alternative processes or treatments lead to differentembodiments. They are not shown in the figures, but will be brieflyexplained below.

In order to further improve the quality of the stiffening layer 7 aswell as the second homo or heteroepitaxial layer 14, further steps canbe conducted. For example, prior to applying the stiffening layer, thefront side surface 2 can be prepared in order to facilitate thenucleation of the epitaxial stiffening layer. Chemical etching usinghydrofluoric acid (HF), plasma etching, standard cleaning one (SC1) withstandard cleaning two (SC2), or any other suitable cleaning techniquemay be used to prepare the front side surface 2. It is also possible touse a thermal oxidation technique and oxide removal before implantingions in order to remove surface defects from the base substrate.

It is also possible to use at least one additional layer as a bufferlayer in order to improve the quality of the stiffening layer 7. Suchadditional layers grown on the substrate 1 relieve any stress that mightoccur inside the stiffening layer. Typical materials that could beutilized are aluminum nitride, gallium nitride, aluminum gallium nitrideor a combination thereof. Sophisticated buffer techniques, like lateralovergrowth or the like, can also be applied. The same kind of additionallayers can also be applied between the epitaxial substrate 10 and thedesired homo or heteroepitaxial layer 14. It is also possible to growthe additional layers before and/or after the implantation step. In afurther variant, the implanting step results in placing the weakenedzone inside one of the additional layers, wherein the additional layerswere grown prior to implantation. After growth of the homo- orheteroepitaxial layer 14, further layers can be grown on the substrate.

In a preferred implementation, at least process steps 1 c to 1 e takeplace in one apparatus without moving the epitaxial substrate 10, thusreducing the risks of introducing contamination or scratches on theepitaxial substrate. Alternatively, if it is advantageous for aparticular planned epitaxial film process, it is possible to separatethe epitaxial substrate 10 from the remainder of the substrate 11 afterstep 1 d. In this case, the epitaxial substrate must be sufficientlythick for it to be handled and moved from one apparatus to another.

The homo or heteroepitaxial films 14 that can be obtained by utilizingthe present methods are in particular films made out of gallium nitride,silicon germanium, aluminum nitride or indium nitride. An example of thefabrication of an epitaxial gallium nitride film on a silicon carbidebase substrate is described below.

In an example, a base substrate is provided of a mono-crystallinesilicon carbide of a 6H or 4H polytype with silicon terminated surface,or 3C (three cubic) silicon carbide. Alternatively, the process couldalso be applied to a base substrate made out of sapphire. Hydrogen ionsare implanted through the front side surface 2 of the base substrate 1with a dose between 2 and 8×10¹⁶ H+/cm² with an energy between 30 and210 KeV. In particular, a dose of 5×10¹⁶ H+/cm² with an energy of 120KeV may be used. As described above, a sacrificial layer 15 can be usedto prevent contamination. In the case of silicon carbide it is possibleto grow a silicon dioxide layer that is a few hundred Å thick by using athermal process. After implantation, the silicon dioxide can be removedby using hydrofluoric acid (HF), for example, in a ten percentconcentration. A 20 minute period of time is usually sufficient toremove the oxide layer.

In the next step, gallium nitride is deposited on the implanted siliconcarbide base substrate by means of MBE at a temperature of less than800° C. This first epitaxial layer corresponds to the stiffening layer 7and has a thickness of some microns. Alternatively, prior to depositinggallium nitride, a buffer layer could be provided. Another alternativeis to clean the surface prior to deposition by chemical etching by usinghydrofluoric acid, or by plasma etching, or by utilizing a SC1 with SC2cleaning technique.

The sample is next heated to a temperature of 900° C. or more, and afracture between the epitaxial substrate 10 and the remainder of thesilicon carbide base substrate 11 occurs. Then a second epitaxial growthstep occurs, wherein gallium nitride is grown to a thickness of up toabout 200-300 micrometers, whereby the growth speed can be relativelyhigh, for example, 50 μm per hour. This growth step takes place attemperatures of more than 900° C., which improves the growth conditionsfor obtaining an epitaxial film with improved crystalline quality.

Finally, the substrate and the newly grown gallium nitride layer coolback to room temperature, and it is possible to separate,the galliumnitride film from the substrate. Using chemical means, the back side ofthe gallium nitride can be cleaned, to remove any remaining siliconcarbide. The original silicon carbide base substrate will be recycled bypolishing its surface, for example, by using chemical-mechanicalpolishing.

1. A method for fabricating an epitaxial substrate, comprising:providing a crystalline or mono-crystalline base substrate; implantingatomic species into a front face of the base substrate to a controlledmean implantation depth to form a zone of weakness within the basesubstrate that defines a sub-layer; growing a stiffening layer on afront face of the base substrate by using a thermal treatment in a firsttemperature range, the stiffening layer having a thickness sufficient toform an epitaxial substrate for further processing; and detaching thestiffening layer and the sub-layer from the base substrate by using athermal treatment in a second temperature range higher than the firsttemperature range, to obtain the epitaxial substrate and a remainder ofthe base substrate, the epitaxial substrate being suitable for use ingrowing high quality homoepitaxial or heteroepitaxial films thereon. 2.The method of claim 1 wherein the atomic species are at least one ofhydrogen ions and rare gas.
 3. The method of claim 1 wherein theimplanting step occurs before growing the stiffening layer.
 4. Themethod of claim 1 wherein the sub-layer is less than approximately 5 μmthick.
 5. The method of claim 1 wherein the first temperature rangecomprises temperatures from about room temperature to about 900° C. andthe second temperature range comprises temperatures from about 900° C.to about 1100° C.
 6. The method of claim 1 which further comprisesproviding a sacrificial layer on the front face of the base substrateprior to implanting atomic species.
 7. The method of claim 6 wherein thesacrificial layer is a thin silicon dioxide (SiO₂) layer.
 8. The methodof claim 6 which further comprises removing the sacrificial layer beforeproviding the stiffening layer.
 9. The method of claim 1 which furthercomprises growing the stiffening layer by at least one of epitaxialgrowth, molecular beam epitaxy (MBE), metal-organic chemical vapordeposition (MOCVD), hydride vapor phase epitaxy (HVPE) or by sputtering.10. The method of claim 1 wherein the stiffening layer is in the rangeof approximately 5 μm to at least approximately 50 μm thick.
 11. Themethod of claim 1 which further comprises pre-treating the surface ofthe base substrate prior to crowing the stiffening layer by using atleast one of HF etching, plasma etching, or a standard cleaningtreatment.
 12. The method of claim 1 which further comprises providingat least one additional layer on top of the stiffening layer or betweenthe base substrate and the stiffening layer.
 13. The method of claim 12wherein the additional layer between the base substrate and a stiffeninglayer is a buffer layer, the buffer layer being made of at least one ofAlN, GaN, AlGaN or a combination thereof.
 14. The method of claim 12further comprising providing at least two additional layers on top ofthe base substrate, and wherein at least one of the additional layers isprovided prior to implanting atomic species.
 15. The method of claim 14which further comprises implanting the atomic species into the at leastone additional layer to create a weakened zone inside the at least oneadditional layer.
 16. The method of claim 1 which further comprisespolishing a surface of the remainder of the base substrate afterdetaching the epitaxial substrate such that the base substrate issuitable for reuse.
 17. The method of claim 1 wherein the base substrateis made of at least one of silicon, silicon carbide, sapphire, galliumarsenide, indium phosphide (InP) or germanium (Ge).
 18. The method ofclaim 1 wherein the stiffening layer is epitaxially grown and is made ofthe same material as an epitaxial film to be grown on the epitaxialsubstrate in a subsequent fabricating step.
 19. The method of claim 1wherein the stiffening layer has a crystalline structure and a thermalexpansion coefficient similar to that of an epitaxial film to be grownon the epitaxial substrate in a subsequent fabricating step.
 20. Themethod of claim 1 wherein the stiffening layer is made out of at leastone of gallium nitride (GaN), aluminum nitride (AlN), indium nitride(InN), silicon germanium (SiGe), indium phosphite (InP), galliumarsenide (GaAs) or alloys made out of those materials.
 21. The method ofclaim 20 wherein the alloys include at least one of AlGaN, InGaN, InGaAsor AlGaAs.
 22. The method of claim 1 wherein a backside surface of theepitaxial substrate created after the detaching step has a surfaceroughness in a range of approximately 20 to about 200 Å RMS.
 23. Themethod of claim 1 further comprising growing a homoepitaxial layer or aheteroepitaxial layer on the epitaxial substrate.
 24. The method ofclaim 23 which further comprises growing the homoepitaxial layer or theheteroepitaxial layer by using a thermal treatment in a thirdtemperature range that is higher than the first temperature range. 25.The method of claim 24 wherein the third temperature range comprisestemperatures greater than about 900° C.
 26. The method of claim 23wherein the homoepitaxial layer or the heteroepitaxial layer is made outof at least one of GaN, SiGe, ALN or InN.